1. Field of the Invention
The present invention relates to a laminated inductor and, more particularly, to a laminated inductor which is constructed to be incorporated in a high-frequency electronic apparatus.
2. Description of the Related Art
A conventional laminated inductor is shown in FIGS. 6A and 6B. As seen in FIG. 6A, a laminated inductor 81 includes a plurality of insulating sheets 83 having coil inductors 82 disposed on the insulating sheets 83. The insulating sheets 83 are laminated and fired with protective sheets 84 and 85 placed on the opposite sides of the laminated insulating sheets 83. The coil conductors 82 are connected to each other via relay via holes 86 formed in the insulating sheets 83, thereby forming a helical coil 90. The two ends of the helical coil 90 are respectively connected to external input and output electrodes 91 and 92 provided on the opposite ends of the laminated inductor 81 on the left and right sides as viewed in FIG. 6B. This connection is made via lead-out via holes 87 respectively formed in the protective sheets 84 and 85. The input and output external electrodes 91 and 92 are arranged to extend perpendicular to the axial direction of the coil 90 and the direction of stacking of the sheets 83 to 85 to improve an insertion loss characteristic in a high-frequency band by reducing a stray capacitance.
In the conventional laminated inductor 81 shown in FIGS. 6A and 6B, however, the area of contact between each coil conductor 82 and the corresponding insulating sheet 83 in an outer portion of the laminated inductor 81 along the periphery of the inductor body is large since the coil conductor 82 extends parallel to the edges of the insulating sheet 83. Ordinarily, the strength of adhesion between coil conductors 82 and insulating sheets 83 is small. Therefore, the mechanical strength of the inductor 81 is considerably small in an outer portion of the inductor 81 where the area of contact between the coil conductors 82 and the sheets 83 is substantially large. As a result, the inductor 81 can break or split easily at locations between the coil conductors 82 and the insulating sheets 83 in the outer portion when an external force F is applied to the inductor 81 in a direction perpendicular to the direction of stacking of the sheets 83 to 85.
Further, the proportion of a size of each coil conductor 82 relative to the size of a respective insulating sheet 83 upon which it is disposed at an outer portion of the inductor 81 is large and can increase significantly if the gap between the coil conductor 82 and one edge of the insulating sheet 83 is reduced due to, for example, a cutting position error occurring at the time of cutting out the inductor block from a mother laminated member, a printing error occurring at the time of forming the coil conductor 82 on the insulating sheet 83, or an error in the superposed position of the insulating sheet 83, resulting in a considerable reduction in the mechanical strength of the inductor 81.
To solve these problems, the gaps between each coil conductor 82 and the edges of the insulating sheet 83 may be increased by reducing the diameter of the helical coil. In such a case, however, inductance L becomes smaller. If the number of turns of the helical coil is increased to compensate for the reduction in inductance L, the number of insulating sheets 83 becomes larger resulting in an increase in manufacturing cost. Alternatively, the width of the pattern of the coil conductor 82 may be reduced to reduce the area of contact between the coil conductor 82 and the insulating sheet 83 in an outer portion of the inductor 81 along the periphery of the inductor body. Then, the DC resistance of the coil conductor 82 is increased, resulting in a deterioration of the efficiency of the inductor 81.